Heretofore, there have been known various types of emulsification/dispersion apparatuses. Irrespective of the types, such as rotary and high-pressure types, all of these emulsification/dispersion apparatuses are intended to perform emulsification/dispersion while giving a high shearing force to a liquid. For example, a high-pressure type homogenizer is designed to perform emulsification/dispersion by converting a pressure into a jet flow, and bringing the jet flow into collision with a wall or reversing its course to convert the kinetic energy of the jet flow into shear energy.
If a liquid receives such a high shearing force in a site having some heterogeneity or unevenness, such as difference in pressure or flow velocity (under off-balanced conditions), air dissolved in the liquid or air remaining in the system will become bubbles or cause a bubbling phenomenon to undesirably form large particles. In this context, a technique of introducing a backpressure has been employed to prevent the bubbling from occurring. For controlling the backpressure, a Gaulin-type homogenizer is provided with a two-way valve, and a microfluidizer is provided with a backpressure chamber.
In conjunction with recent developments of an apparatus capable of injecting higher energy in response to the need for higher emulsification/dispersion performance, the bubbling phenomenon comes to the front as a serious problem. While bubbling likely to occur within an apparatus can be suppressed by increasing a backpressure, another bubbling will be caused by pressure drop immediately after the liquid is discharged from the apparatus.
In case where a target material is power or fine particles, upon occurrence of the bubbling, the resulting bubbles attached onto the surfaces of the particles cause poor wettability, and an aerosol is liable to be formed even in an emulsion state. The bubbles also absorb energy to cause energy loss, and oxidization in a treated material. For example, in case of unsaturated fatty acid, an oxidative reaction under a high temperature causes deterioration in product quality. Moreover, if the amount of an emulsifying agent and/or a dispersing agent is increased to reduce the interfacial tension of aquatic systems, small bubbles generated in the shear region will further attach onto the interface to cause the deterioration in release of the bubbles.
As above, the conventional concept is directed to injecting higher energy so as to facilitate micronization, and prevent bubbling by means of increased backpressure, or is based on an idea giving greater importance to the upstream of an emulsification/dispersion process. Such an approach cannot effectively prevent the occurrence of bubbling.